EP1165445A1

EP1165445A1 - Water purification

Info

Publication number: EP1165445A1
Application number: EP00911924A
Authority: EP
Inventors: Anthony M. Wachinski
Original assignee: Pall Corp
Current assignee: Pall Corp
Priority date: 1999-02-24
Filing date: 2000-02-24
Publication date: 2002-01-02
Also published as: AU3373800A; WO2000050346A1; WO2000050346A8; CA2364247A1

Abstract

Systems and methods for water purification are disclosed. The system includes an inlet for untreated water, an outlet for purified water, a filter assembly having a filter medium interposed between the inlet and the outlet, and a chlorine dioxide source. Water is purified by contacting chlorine dioxide with water and passing the water through a filter medium. The systems and methods are particularly applicable for the purification of industrial process water, and municipal surface water and ground water for potable use.

Description

WATER PURIFICATION

The present application claims the benefit of priority of United States Patent Application No. 60/121,652 filed on February 24, 1999. United States Patent Application No. 60/121,652 and all other patent applications, patents, and publications listed in the present application are incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The invention relates to water purification; in particular, to the use of chlorine dioxide in water purification.

BACKGROUND OF THE INVENTION

Filter media have been used for industrial water purification, and for municipal water purification, e.g. surface and ground water purification, to remove particulate matter, viruses, microorganisms, and dissolved materials. However, such filters have long had the drawback of fouling caused by, for example, the accumulation of parti culates and organic matter, or the growth of a biofilm on the filter medium. The biofilm may be formed by natural organic matter, inorganic matter, and microorganisms accumulating on the surface or within the pores of the filter medium, causing fouling. The fouling of the filter medium may cause a reduction in flow rate or flux of water through the filter medium. Accordingly, as the filter medium fouls, the pressure that must be applied across the filter medium to force water through, referred to as the transmembrane pressure (TMP), must be increased. However, there are limits to how high the pressure may be applied before causing damage to the filter medium. When the pressure reaches its upper limit, filtration must be suspended and the filter medium must be cleaned. Cleaning of the filter medium at least partially restores the flux through the filter medium. The decrease in flux through the filter medium caused by fouling and the required frequency to stop filtration and clean the filter medium results in an inefficient and expensive process.

Ozone has been used to disinfect water and to reduce fouling of a filter medium in water purification. However, ozone may produce harmful byproducts in the resulting purified water, such as aldehydes and ketones. In addition, ozone has not been used successfully in conjunction with filter media made from a polymeric material because of the oxidizing strength of ozone.

Chlorine and chlorine derivatives, such as chlorine dioxide and chloroamines, have been used in water purification to remove objectionable tastes and odors, to oxidize iron and manganese for precipitation, for disinfection, and for color removal. However, there has been increasing concern about byproducts, known as disinfection byproducts (DBPs), which may be produced during the purification of water when chlorine or chlorine derivatives are used. Disinfection byproducts may include, for example, chlorine, chlorate ions, chlorite ions, bromate ions, trihalomethanes (e.g. chloroform, bromodichloromethane, dibromochloromethane, and bromoform), and haloacetic acids (e.g. dichloroacetic acid and trichloroacetic acid). In the United States, regulations were recently promulgated which limit the amount of DBPs that may be present in purified drinking water due to the potential health effects of DBPs. For example, the U.S. Environmental Protection Agency has established a limit of 0.080 mg/L total trihalomethanes and 0.060 mg/L for haloacetic acids for drinking water. However, chlorine dioxide currently used in water purification may result in the purified water exceeding these DBP limits.

SUMMARY OF THE INVENTION

An aspect of the invention includes a method of purifying water. The method comprises contacting chlorine dioxide with water and passing the water through a filter medium. The water may be passed through a filter medium with a nominal pore size of about 2 microns or less. The chlorine dioxide may be contacted with the water at a concentration of about 10 parts per million or less. Preferably, the chlorine dioxide used is ultra-high purity chlorine dioxide. Another aspect of the invention includes a system for purifying water. The system comprises an inlet for untreated water, an outlet for purified water, a filter assembly, and a chlorine dioxide source. The filter assembly is interposed between the inlet and the outlet and comprises a filter medium. The filter medium preferably has a nominal pore size of about 2 microns or less. The chlorine dioxide source is interposed between the inlet and the outlet and is in fluid communicable relationship with the untreated water to allow contacting of chlorine dioxide with untreated water. Preferably, the chlorine dioxide source allows contacting of chlorine dioxide with untreated water at a concentration of about 10 parts per million or less. Preferably, the chlorine dioxide is ultra-high purity chlorine dioxide.

It has been discovered that chlorine dioxide may be used in a water purification system, such as an industrial process water purification system, or a municipal water purification system, e.g., for surface water or ground water, to purify the water and enhance the flux of water across a filter medium, particularly a polymeric filter medium. The contacting of chlorine dioxide with water upstream of a filter medium continuously cleans the filter medium, retarding or preventing the accumulation of organic matter and the development of a biofilm on the filter medium. As a result, fouling of the filter medium is reduced or prevented altogether, allowing lower transmembrane pressures to be used and reducing or eliminating the need to stop filtration and clean the filter medium. Because the chlorine dioxide oxidizes organic matter, including organic compounds such as odor- or flavor-causing compounds, and kills microorganisms such as viruses, bacteria, algae and protozoa such as Cryptosporidium and Giardia, and filter media, particularly those media with a nominal pore size of about 2 microns or less, filter such microorganisms and natural organic matter, both agents act in tandem to purify the water. There has been discovered a synergy between the use of chlorine dioxide and filter media, wherein the combined use of a filter medium and chlorine dioxide results in a more efficient process and system than if they were used separately. It has also been discovered that chlorine dioxide may be effectively used with polymeric filter media in water purification, particularly polymeric filter media that are resistant to chlorine dioxide oxidation or corrosion.

It has also been discovered that high-purity chlorine dioxide, preferably ultra-high purity chlorine dioxide, provides for additional advantages over standard purity chlorine dioxide for use in the purification of water. Ultra-high purity chlorine dioxide may even further enhance the flux of water through a filter medium, particularly for a filter medium with nominal pore size of about 2 microns or less, over standard purity chlorine dioxide. Also, the use of high purity chlorine dioxide, preferably ultra-high purity chlorine dioxide in the water purification system, particularly when used in concentrations in the feed or untreated water at about 10 parts per million or less, results in the purified water having lower disinfection byproducts. By using ultra-high purity chlorine dioxide at a 4

concentration of about 10 parts per million or less in tandem with a filter medium of nominal pore size of about 2 microns or less, purified water is obtained more efficiently, less expensively, and is safer for the public to drink.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a portion of a water purification system according to the present invention including a prefilter, a dead-end filter assembly, and chlorine dioxide contact points.

Figure 2 illustrates a portion of a water purification system according to the present invention including a prefilter, a cross-flow or excess recirculation filter assembly, and chlorine dioxide contact points.

Figure 3 illustrates a portion of a water purification system according to the present invention including a prefilter, a dynamic filter assembly, and chlorine dioxide contact points.

DETAILED DESCRIPTION

The present invention relates to the use of chlorine dioxide and filtration in the purification of water. In particular, the present invention may be used to treat industrial process water, or municipal water, such as ground water or surface water. In an aspect of the invention, chlorine dioxide may be contacted with untreated water and the water may be passed through a filter medium. The contacting of chlorine dioxide with untreated water purifies the water and enhances the flux of the water through a filter medium.

Chlorine dioxide may be contacted with water in a concentration that is sufficient to enhance the flux of water through a filter medium. For example, a concentration as low as 1 part per million of chlorine dioxide in water, or even lower, may be used to enhance the flux of water through a filter medium. Flux may be defined as the flow rate of the water through the filter medium divided by the surface area of the filter medium.

In an embodiment of the invention, the concentration of chlorine dioxide is low enough so that the level of disinfection byproducts, such as chlorate ion and chlorite ion, in the purified water do not exceed applicable standards. For example, the amount of chlorine dioxide that is contacted with the water may be less than about 10 parts per million (ppm), preferably less than about 7 ppm, more preferably less than about 4 ppm, even more preferably less than about 3 ppm, even more preferably less than about 2 ppm, even more preferably about 1 ppm or less. However, higher concentrations of chlorine dioxide may be contacted with the water provided that the amount of disinfection byproducts does not exceed acceptable levels in the purified water or such byproducts are subsequently reduced to desired limits in the purified water.

The chlorine dioxide may be contacted with the untreated water in a variety of ways. For example, chlorine dioxide may be dissolved in water in a concentrated form and be allowed to contact the untreated water. Diluted or undiluted chlorine dioxide may be added to or injected into the untreated water stream. Alternatively, the chlorine dioxide as a gas may be contacted with, e.g. bubbled, into the untreated water. Preferably, the chlorine dioxide is contacted with the untreated water on a continuous basis during purification. Chlorine dioxide in various grades of purity may be used in the present invention. For example, in embodiments of the invention, standard purity, high-purity, or ultra-high purity chlorine dioxide may be used. The purity of chlorine dioxide may be determined by the amount of byproducts present along with the chlorine dioxide, such as chlorine, chlorate ion, and/or chlorite ion. Standard purity chlorine dioxide may include about 30 to about 200 mg/L chlorine, greater than about 80 mg/L of chlorate ion, and/or greater than about 60 mg/L chlorite ion, per 1000 mg/L chlorine dioxide. High purity chlorine dioxide may include about 3 to about 30 mg/L chlorine, about 3 to about 80 mg/L chlorate ion, and/or about 10 to about 60 mg/L chlorite ion, per 1000 mg/L chlorine dioxide. Ultra-high purity chlorine dioxide may include about zero to about 3 mg/L chlorine, about zero to about 3 mg/L chlorate ion, and/or about zero to about 10 mg/L of chlorite ion, per 1000 mg/L chlorine dioxide.

Chlorine dioxide in various grades of purity, such as standard purity, high purity, and ultra-high purity, are available from Sterling Chemical Corporation. For example, ultra-high purity chlorine dioxide may be obtained from Sterling Chemical Corporation under the ECF trade designation. In some embodiments of the invention, use of higher purity chlorine dioxide is preferred because it purifies the water, results in an increased flux of water across the filter medium, and produces fewer disinfection byproducts in the purified water. Ultra-high purity chlorine dioxide is especially preferred because it can be used at high concentrations, e.g., from about 4 to about 10 ppm, with fewer disinfection byproducts.

Various forms of filter elements may be used with the present invention. For example, the filter element or filter elements may comprise filter media in sheet form, illustratively in a plate and frame module, e.g. as a fibrous sheet, a sintered metal plate, or a porous film. Alternatively, the filter element may be configured as a cylindrical element, e.g., a dead end filter element including a ceramic candle filter, a fibrous mass, a hollow pleated configuration, such as a straight, radial pleat design or a laid-over configuration, as disclosed, e.g., in U.S. Patent No. 5,543,047, or a cross flow filter element, such as disclosed, e.g., in International Application No. PCT/US99/20509. Some filter elements, such as Pall Corporation's Septra XS filter element, may be used in a dead end mode or a cross-flow mode. Also, the filter element may comprise a composite including a filter medium or filter media and additional layers that are in fluid communication with the filter medium or media, including drainage and/or support layers and/or cushioning layers. Other suitable configurations include spiral or tubular modules. Preferably, the filter element comprises hollow fibers contained in a hollow fiber module.

Various types of filter media may be used with the present invention. The filter media used in the filter element may include any material capable of forming a porous structure suitable for filtering water, including porous metal media, porous ceramic media, porous mineral media, porous media comprising organic and/or inorganic fibers such as carbon and/or glass fiber media, and/or porous polymeric media. The filter media may include fibrous media such as a mass of fibers, fibrous mats, woven or non- woven sheets, and fibrous depth filters made by a variety of means including melt-blowing, Fourdrinier deposition, or air laying fibrous materials. In addition, the filter media may include hollow fibers or a porous film or membrane, e.g. isotropic or anisotropic membranes such as asymmetric or composite membranes.

Preferably, the filter media is made of a material which is substantially resistant to chlorine dioxide oxidation or corrosion. Polymeric media, particularly polymeric media resistant to oxidation by chlorine dioxide are preferred, such as polyvinylidene fluoride (PVDF) (e.g., as available from Pall Corporation under the trade designation Microza). Polyethersulfone (PES), polysulfone (PS), polytetrafluoroethylene (PTFE) or polycarbonate (PC) may also be useful materials for a filter medium in the present invention.

The porous filter media of the present invention are not restricted to any particular pore sizes or structures. The pore size used depends on the composition of the water to be purified and the desired purity level of the water. Preferably, the pore size of the filter medium is small enough to capture particulates and microorganisms such as algae, bacteria, viruses, and or protozoa such as Cryptosporidium and Giardia. Microporous and ultraporous media are preferred, although nanofiltration membranes or reverse osmosis membranes may be used. The nominal pore size of the filter medium may be about 2 microns or less, preferably about 1 micron or less, most preferably about 0J microns or less. The type of filter assembly utilized in the invention is not particularly limited. For example, any dead-end filtration assembly, such as an assembly of cylindrical dead-end filter elements or cartridges or dead-end hollow fiber modules, or a cross-flow filtration assembly, such as an assembly of cylindrical cross-flow filter elements, a cross-flow module having a stack of flat filter elements, or an assembly of cross-flow hollow fiber modules, may be used. Alternatively, a dynamic filter assembly, such as those disclosed, for example, in International Publications No. WO 95/00231, No. WO 97/02087, and No. WO 97/13571, may be used. The flow through the filter element may be outside-in, where untreated water contacts the outside surface(s) of a filter medium, with filtrate or permeate passing through the filter medium to the inside surface(s) of the filter medium. Alternatively, the flow through the filter element may be inside-out, where the untreated water contacts the inside surface(s) of a filter medium, with filtrate or permeate passing through the filter medium to the outside surface(s) of the filter medium.

The filter assembly may be used in any water purification system. Examples of suitable purification systems include a batch system with an open loop, a batch system with a closed loop, a single-stage continuous system, a multistaged arrangement with recirculation, and a multistaged arrangement without recirculation, as described, for example, in Water Treatment Membrane Processes, American Water Works Association Research Foundation et al., 1995, pages 2.22-2.24. In the purification system, a prefilter may be used upstream of the filter element to remove larger particulate matter. Such a prefilter may have a larger pore size than the filter medium, e.g. on the order of 400 microns in nominal pore size.

The chlorine dioxide may be contacted with water in a variety of locations. For example, chlorine dioxide may be contacted with untreated water upstream of the filter medium. If a prefilter is used, the chlorine dioxide may be added upstream of the prefilter and/or downstream of the prefilter. If a cross-flow system is used, chlorine dioxide may be added upstream of the filter medium. If the retentate is recirculated, chlorine dioxide may be added to a portion of a retentate stream that is recycled to the upstream portion of the filter medium.

An embodiment of the invention includes a system for purifying water. The system includes an inlet for untreated water and an outlet for purified water. A filter assembly is interposed between the inlet and the outlet and comprises a filter medium. A chlorine dioxide source is in fluid communicable relationship with the water to allow contacting of chlorine dioxide with the water.

The inlet for untreated water may be any suitable means for allowing untreated water to travel to the filter assembly. For example, the inlet may be a conduit, pipe, or manifold which is in fluid communication with the filter assembly. Similarly, the outlet for treated or purified water may be any suitable means allowing the purified water to exit the purification system. For example, the outlet may be a conduit, pipe, or manifold in fluid communication with the filter assembly. The filter assembly may include an inlet for water to be treated, an outlet for treated water, and one or more filter elements, each including a filter medium. The filter medium preferably has a nominal pore size of about 2 microns or less which enables filtration of particulates and or microorganisms with an acceptable applied transmembrane pressure across the filter medium. A chlorine dioxide source is preferably in fluid communicable relationship with the water. The chlorine dioxide may be generated on site and supplied to the water. Alternatively, chlorine dioxide may be generated off-site and stored as a gas or dissolved in a liquid, e.g. water, in concentrated form in a tank or other container, and supplied to the water. As discussed above, the chlorine dioxide source may generate or contain standard purity, high purity, or ultra-high purity chlorine dioxide.

By "fluid communicable relationship" it is meant that the chlorine dioxide source is in fluid communication with the untreated water so that chlorine dioxide may be contacted with the water. For example, the chlorine dioxide source, generating or containing chlorine dioxide as a gas or dissolved in a liquid, e.g. water, may be coupled to the water stream by an injector, a conduit, a pipe, or a manifold. Alternatively, the chlorine dioxide gas or chlorine dioxide dissolved in a liquid may be contacted with the water by passing it through a permeable or semipermeable membrane into the water.

The chlorine dioxide source may be in fluid communication with the water in a variety of locations in the purification system. For example, if a prefilter is placed upstream of the filter assembly, the chlorine dioxide source may be in fluid communication with untreated water upstream of the prefilter and/or downstream of the prefilter. Alternatively or additionally, the chlorine dioxide source may be in fluid communication with water immediately upstream of the filter element.

For filter assemblies with a retentate outlet and a permeate outlet, e.g. in cross-flow filter devices, the chlorine dioxide source may be in fluid communication with the retentate that is recycled back upstream of the filter medium. Alternatively or additionally, the chlorine dioxide source may be in fluid communication with the water at the point of mixing of the retentate and the water passing from upstream of the filter medium towards the filter medium, and/or after the water streams have mixed together.

The contacting of the water and the chlorine dioxide may include any suitable means for thoroughly contacting and reacting the chlorine dioxide with the water and the substances in the water. For example, the chlorine dioxide and the water may be mixed using a mixer, e.g. a mechanical or static mixer. Alternatively or additionally, the chlorine dioxide may be added to water a sufficient distance from the filter medium to allow for thorough contacting and reaction prior to filtration. The chlorine dioxide source is capable of delivering a concentration of chlorine dioxide to the water to enhance the flux across a filter medium, as well as to minimize the production of disinfection byproducts. For example, the chlorine dioxide source may deliver chlorine dioxide to the water at a concentration of about 10 ppm or less, preferably about 7 ppm or less, even more preferably about 4 ppm or less, even more preferably at about 3 parts ppm or less, even more preferably at about 2 ppm or less, most preferably at about 1 ppm or less. The transmembrane pressure (TMP) that may be applied across the filter medium depends upon the filter system, desired flow rate, and degree of fouling of the filter medium. For example, using a filter module comprising hollow fibers and being about 3-6 inches in diameter and 24 inches to 2 meters long, the application of a TMP of about 5 to 30 psi may result in a flow rate of about 5-20 gallons per minute. Because chlorine dioxide reduces or prevents fouling of the filter medium, the flux of water through the filter medium may be increased for a given TMP. Also, the TMP that may be applied during filtration may be lower and may increase more slowly, if at all, to maintain a certain rate of flux of water through the filter medium. As a result, the limit of TMP that may be used to force water through the filter medium may be reached more slowly, if at all. Accordingly, not only is the flux of water increased but also filtration may be performed for longer periods of time before stopping to clean the filter medium, if cleaning is required at all.

Figure 1 illustrates an embodiment of a water purification system 100 of the present invention including a prefilter 20 and a dead-end filter assembly 40. Untreated water 10 enters upstream of the prefilter 20 and passes through the prefilter 20, e.g. a 400 micron pore size disposable strainer. The prefiltered water 30 then enters the dead-end filter assembly 40 wherein water 30 passes through the filter medium of the filter assembly 40, forming purified water or filtrate 50, while particulates and microorganisms do not pass through the filter medium. As illustrated in Figure 1, chlorine dioxide may be contacted with the untreated water at one or more locations in the water purification system. For example, chlorine dioxide may be contacted with the untreated water 10 upstream of the prefilter 20, illustrated by contact point 60. Alternatively or additionally, chlorine dioxide may be contacted with the prefiltered water 30 downstream of the prefilter and upstream of the dead-end filter 40, illustrated by contact point 70. The chlorine dioxide added to untreated water 10 upstream of the prefilter 20, and/or to prefiltered water 30 downstream of the prefilter 20 and upstream of the filter 40, purifies the water and increases the flux of purified water across the membrane.

Figure 2 illustrates another embodiment of a water purification system of the present invention. The embodiment comprises a water purification system 200 with a prefilter 210 and a cross-flow or excess recirculation filter assembly 220. Untreated water 205 enters upstream of prefilter 210 and passes through the prefilter 210. The prefiltered water 215 is then passed tangentially across the surface(s) of the filter medium of the filter assembly 220. Purified water or permeate 225 passes through the filter 220 and proceeds to permeate outlet 230. Retentate 235 flows through retentate outlet 240. At least a portion of the retentate 235 may be recycled back and the recycled retentate 275 may be mixed with prefiltered water 215, e.g. downstream of the prefilter 210 and upstream of the filter assembly 220, illustrated by path 245 and point 250. Retentate not recycled back to the upstream portion of the filter medium is illustrated by non-recycled retentate stream 280.

As illustrated in Figure 2, chlorine dioxide may be contacted with the water at one or more locations of the water purification system. For example, chlorine dioxide may be contacted with untreated water 205 upstream of the prefilter 210, illustrated by point 255. Alternatively or additionally, chlorine dioxide may contacted with prefiltered water 215 downstream of prefilter 210 and upstream of the cross-flow filter 220, illustrated by point 260. Alternatively or additionally, chlorine dioxide may be contacted with retentate 235, illustrated by point 285. Alternatively or additionally, chlorine dioxide may be contacted with the recycled retentate 275 before mixing with the prefiltered water 215, illustrated by point 265. Alternatively or additionally, chlorine dioxide may be contacted with the combined recycled retentate 275 and prefiltered water 215 upstream of the filter medium of filter 220, illustrated by point 270. The addition of chlorine dioxide purifies the water and enhances the flux of water across the filter medium.

Figure 3 illustrates another embodiment of a water purification system of the present invention. The embodiment comprises a water purification system 300 with prefilter 310 and a dynamic filter assembly 320. Untreated water 305 enters upstream of prefilter 310 and passes through the prefilter 310. The prefiltered water 315 is then passed along the dynamic filter assembly 320, for example one or more moving or stationary elements. Purified water or permeate 325 passes through filter 320 and proceeds to permeate outlet 330. Retentate 335 flows through retentate outlet 340. At least a portion of the retentate 335 may be recycled back and the recycled retentate 375 may be mixed with prefiltered water 315, e.g. downstream of the prefilter and upstream of dynamic filter assembly 320, as illustrated by path 345 and point 350. Retentate not recycled back to the upstream portion of the filter medium is illustrated by non-recycled retentate stream 380. As illustrated in Figure 3, chlorine dioxide may be contacted with the water at one or more locations of the water purification system. For example, chlorine dioxide may be contacted with untreated water 305 upstream of the prefilter 310 and the dynamic filter assembly 320, as illustrated by point 355. Alternatively or additionally, chlorine dioxide may be contacted with prefiltered water 315 downstream of prefilter 310 and upstream of dynamic filter assembly 320, as illustrated by point 360. Alternatively or additionally, chlorine dioxide may be contacted with retentate 335, illustrated by point 385. Alternatively or additionally, chlorine dioxide may be contacted with the recycled retentate 375 before mixing with the prefiltered water 315, as illustrated by point 365. Alternatively or additionally, chlorine dioxide may be contacted with the combined recycled retentate 375 and prefiltered water 315 upstream of the filter medium of the dynamic filter 320, as illustrated by point 370. The addition of chlorine dioxide purifies the water and enhances the flux of water across the filter medium.

By adding chlorine dioxide upstream of a filter medium during filtration, particularly for a filter medium of nominal pore size of about 2 microns or less, the buildup of organic matter (often referred to as total organic carbon, or TOC) and biofilm on the filter media is reduced or prevented, allowing greater flux of water through the filter medium and less frequent cessation of filtration for cleaning of the filter medium. The addition of chlorine dioxide at a concentration of about 10 parts per million or less to water upstream of a filter medium with nominal pore size of about 2 microns or less has been discovered to work surprisingly well as part of a water purification system. The chlorine dioxide kills microorganisms such as ocysts and oxidizes iron and manganese so that they precipitate in solution. The filter medium traps the metal particulates, other particulates, microorganisms, and other organic matter. Accordingly, the filter medium and the chlorine dioxide work in tandem to purify water, such as ground water and surface water, for municipal use, and industrial process water.

The use of high purity, particularly ultra-high purity chlorine dioxide helps provide for an even greater flux of water through the filter medium over standard purity chlorine dioxide and purifies the water while resulting in lower amounts of disinfection byproducts in the purified water. The use of ultra-high purity chlorine dioxide at a concentration of about 10 parts per million or less in conjunction with a filter medium with nominal pore size of 2 microns or less has been found to be superior in purifying water more efficiently and less expensively than existing water purification systems with a lower resulting amount of potentially harmful disinfection byproducts.

Although the present invention has been described in terms of exemplary embodiments, it is not limited to these embodiments. Alternative features, embodiments, examples, and modifications which would still be encompassed by the invention may be made by those skilled in the art, particularly in light of the foregoing teachings. For example, one or more of the features of any one of the embodiments may be combined with one or more of the features of any of the other embodiments. As one example, the cross- flow filter of the embodiment shown in Figure 2 may be coupled to the purified water outlet of the dead-end filter of the embodiment shown in Figure 1. Further, one or more of the features of any of the embodiments may be modified or omitted. As another example, the prefilter may be omitted from any of the embodiments shown in Figures 1-3. Therefore, the following claims are intended to cover any alternative features, embodiments, examples, modifications, or equivalents which may be included within the spirit and scope of the invention as defined by the claims.

Claims

What is claimed is:

1. A method of purifying water comprising contacting chlorine dioxide with water at a concentration of about 10 parts per million or less and passing the water through a filter medium with nominal pore size of about 2 microns or less.

2. The method of claim 1 wherein contacting the chlorine dioxide with water comprises contacting chlorine dioxide with water at a concentration of about 4 parts per million or less.

3. The method of claim 1 or 2 wherein contacting the chlorine dioxide with water comprises contacting chlorine dioxide with water at a concentration of about 2 parts per million or less.

4. The method of any one of claims 1-3 wherein contacting the chlorine dioxide with water comprises contacting chlorine dioxide with water at a concentration of about 1 part per million or less.

5. The method of any one of the preceding claims wherein passing the water through the filter medium comprises passing the water through a filter medium with nominal pore size of about 1 micron or less.

6. The method of any one of the preceding claims wherein passing the water through the filter medium comprises passing the water through a filter medium with nominal pore size of about 0J microns or less.

7. The method of any one of the preceding claims wherein passing the water through the filter medium includes passing the water through a filter medium of a cross-flow filter assembly.

8. The method of any one of the preceding claims wherein passing the water through the filter medium includes passing the water through a filter medium of a dynamic filter assembly.

9. The method of any one of the preceding claims wherein passing the water through a filter medium includes passing the water tangentially across the filter medium and producing a permeate and a retentate.

10. The method of claim 9 wherein at least a portion of the retentate is recycled and passed through the filter medium.

11. The method of any one of claims 1 -6 wherein passing the water through the filter medium includes passing the water through a filter medium of a dead-end filter assembly.

12. The method of any one of the preceding claims wherein contacting chlorine dioxide with water includes injecting chlorine dioxide into the water.

13. The method of any one of the preceding claims wherein contacting chlorine dioxide with water includes mixing chlorine dioxide dissolved in water as a concentrate into the water.

14. The method of any one of the preceding claims wherein contacting the chlorine dioxide with water includes contacting the chlorine dioxide with surface water.

15. The method of any one of claims 1-13 wherein contacting the chlorine dioxide with water includes contacting the chlorine dioxide with ground water.

16. The method of any one of the preceding claims wherein contacting the chlorine dioxide with water includes contacting high purity chlorine dioxide with the water.

17. The method of any one of claims 1-15 wherein contacting the chlorine dioxide with water includes contacting ultra-high purity chlorine dioxide with the water.

18. A system for purifying water comprising: an inlet for untreated water; an outlet for purified water; a filter assembly interposed between the inlet and the outlet and comprising a filter medium with a nominal pore size of about 2 microns or less; and a chlorine dioxide source interposed between the inlet and the outlet and in fluid communicable relationship with the water to allow contacting of chlorine dioxide with the water at a concentration of about 10 parts per million or less.

19. The system of claim 18 wherein the filter medium is resistant to chlorine dioxide.

20. The system of claim 18 or 19 wherein chlorine dioxide source is in fluid communicable relationship with the water to allow contacting of chlorine dioxide with the water at a concentration of about 4 ppm or less.

21. The system of any one of claims 18-20 wherein the filter medium has a nominal pore size of about 0J microns.

22. The system of any one of claims 18-21 wherein the filter medium comprises a polymeric filter medium.

23. The system of any one of claims 18-21 wherein the filter medium comprises a mineral filter medium.

24. The system of any one of claims 18-21 wherein the filter medium comprises a ceramic filter medium.

25. The system of any one of claims 18-21 wherein the filter medium comprises a metal filter medium.

26. The system of any one of claims 18-25 wherein the filter element comprises hollow fibers.

27. The system of claim any one of claims 18-26 further comprising a prefilter upstream of the filter element.

28. The system of any one of claims 18-27 wherein the chlorine dioxide source comprises an on-site chlorine dioxide generator.

29. The system of any one of claims 18-27 wherein the chlorine dioxide source comprises a container including chlorine dioxide produced off-site.

30. The system of any one of claims 18-29 wherein the chlorine dioxide source includes chlorine dioxide dissolved in water.

31. The system of any one of claims 18-30 wherein the chlorine dioxide source is coupled to the water via a conduit.

32. The system of any one of claims 18-31 comprising a chlorine dioxide contact point upstream of the filter medium.

33. The system of any one of claims 18-32 wherein the filter element includes a retentate outlet and a permeate outlet, and a chlorine dioxide contact point downstream of the retentate outlet.

34. The system of any one of claims 18-33 wherein the chlorine dioxide source comprises a high-purity chlorine dioxide source.

35. The system of any one of claims 18-33 wherein the chlorine dioxide source comprises an ultra-high purity chlorine dioxide source.

36. The system of any one of claims 18-35 wherein the filter element comprises a cross-flow filter element.

37. The system of any one of claims 18-35 wherein the filter element comprises a dynamic filter element.

38. The system of any one of claims 18-35 wherein the filter element comprises a dead-end filter element.

39. The method of any one of claims 1-17 wherein contacting the chlorine dioxide with water includes contacting the chlorine dioxide with industrial process water.